Flight Disruptions
Evaluating schedule reliability
By Jack Hessburg
May 2000
Flight disruptions are unacceptable. The most perishable commodity in aviation is an available seat or ton-mile. Simply put, "No butts in the seats or boxes in the pit - no revenue." Lost revenue due to cancellation or delays resulting in passenger loss to another carrier is never recoverable. Delays and cancellations frustrate passengers and disturb their plans. According to FAA data available for 1994, the calculated annual total operating delay cost to the airlines was $2.5 billion and $7 billion attributable losses to passengers. The average delay was 14.23 minutes. Passengers and shippers expect reliable on-time flights. Delays and cancellations tarnish the image of the airline.
Schedule reliability, sometimes called dispatch reliability, is the measure of the extent of flight disruptions. It is the number of occurrences a flight is away from the gate on time. It is usually measured as a percentage of the total number of scheduled departures or the total for a given airplane type. Departure reliabilities may also be measured against a given station.
What is on-time?
It is usually accepted in the business that "on-time" means any departure within 15 minutes of the published schedule; however, this is not a hard rule as some operators measure on-time differently. One airline uses one minute rather than 15 as its standard. Another uses no tolerance at all. Some don't even measure departure time; rather, they measure on-time arrivals. Arrival reliability actually makes more sense. Passengers and boxes must arrive at a specific time. It really doesn't matter when they depart. A late departure may be irritating to customers, but a late arrival is a true disruption to their plans.
Is 100 percent departure reliability the only acceptable value? Not necessarily. Give me unlimited funds, manpower, and resources and I will give you 100 percent for the factors under my control. This, of course, is not practical. An acceptable schedule reliability figure is a balance between available resources such as equipment, spares, maintenance capability, staffing, schedule patterns, and the like.
Each airline has an acceptable range for schedule reliability within each of its fleets. Acceptability many times is based upon the ability to cross load or down load passengers and cargo to other flights and thus not lose revenue. If you are not losing revenue at "X" percent schedule reliability, why attempt to achieve a higher value? This is frequently tempered by a business decision regarding public image. Some airlines want 100 percent schedule reliability, regardless of the cost.
Managing disruptions
Knowing the cause of a disruption can be an effective management tool.
It allows identification of equipment or spares problems for example. Disruptions are broadly categorized into either non-controllable or controllable. The former is a delay/cancellation due to weather, traffic control, etc. They are essentially out of the direct control of the operator. The latter relates to the inherent reliability of the airplane and its systems; the effective management of the line maintenance operations or the efficient management of support services such as handling, fueling, catering, crew scheduling, etc.
Maintenance disruptions
Maintenance disruptions may be divided into two elements - those affected by maintenance operations or those affected by equipment reliability.
Maintenance operations influence disruptions in many ways. The availability of spares and tooling at a given station, people available, and turnaround time for maintenance are some of the ways.
If you don't have the right tools, the appropriate spares, enough people, and enough time, you will have a delay or cancellation. Reliability of equipment is fixed at the time of design. You can not inspect it into a bad design. It is defined by three factors:
• Components and systems are designed with inherent reliability
• Minimum Equipment List is robust
• The airplane is easily repairable at the gate when a no-go condition occurs.
All of these can be done to a very high degree, but usually at exorbitant cost. So, designers must involve themselves in trade-offs that result in acceptable reliability within reasonable cost. Consequently, in service, stuff happens — airplanes break.
Placing the blame
Flight disruptions are usually charged against a specific discipline
or organization affecting the schedule, for example weather, ATC, catering, crew scheduling, ground handling, maintenance, passenger services, etc. The categorization of these elements is established by the reporting policies of a specific airline. Some are very detailed; others do not categorize a delay at all. For example, one airline I am familiar with has 86 different categories for flight disruptions; another only accounts for delays by individual station rather than by organization or discipline.
Schedule reliability reports are not always a valid index of what really caused the delay. This is an Achilles' heel - accurate reports. There are several factors contributing to accuracy. Interdepartmental rivalries, the "report card" syndrome, possible exposure to regulatory action, pilots vs. mechanics vs. management conflicts, and the thought of being singled out for a delay at the morning blood-letting meeting are a few issues that can skew the accuracy of a report. They are all part of the human condition. Organizations and people have agendas. None of us like to be embarrassed by being a party to a delay. Thus, delay reports many times are the product of creative writing and actions rather than raw facts.
Tricks of the trade
I know of a case where, during a delay, the mechanic stole the keys to the catering truck. This gave him a little more time to fix the airplane
and the delay was charged to catering not maintenance. Now the mechanic and the maintenance department didn't get stuck with the delay. Don't worry about the guy driving the catering truck. I eventually gave him the keys back. Today, truck keys at most stations are attached to the trucks by a very stout piece of 7x19. Another stunt has been to push the airplane back from the gate and then pull it back in. Thus, the airplane blocked out on-time and then had a gate turnback. The problem must be with the airplane and not a maintenance management delay.
The point of all of this is that it is who and how the flight delay is called that affects the outcome. Delay reports can be skewed and misleading. They are a very useful tool for isolating schedule problems, but they must always be read carefully. Sometimes, it is not so much what is said in a report as what is not said. Learn how to read between the lines. Don't accept schedule reliability values at face value.
